function [epst] = Epst(x, y, z, w)
%
% The Epstein Function
%
%DESCRIPTION:
%This function computes the Epstein Function, which is used as a basis
%analutical function in NeQuick for the construction of the ionospheric
%layers.
%
%PROTOTYPE:
% [epst] = Epst(x, y, z, w)
%
%--------------------------------------------------------------------------
% INPUTS:
%   x          [1x1]       Peak Amplitude            [-]
%   y          [1x1]       Peak Height               [-]
%   z          [1x1]       Thickness Around the Peak [-]
%   w          [1x1]       Height Dependent Variable [-]
%--------------------------------------------------------------------------
% OUTPUTS:
%   epst       [1x1]       Epstein Fun. Result       [-]
%--------------------------------------------------------------------------
%
%NOTES:
% (none)
%
%CALLED FUNCTIONS:
% (none)
%
%UPDATES:
% (none)
%
%REFERENCES:
% [1] "Ionospheric Correction Algorithm for Galileo Single-Frequency Users"
%      - European GNSS (Galileo) Open Service
%
%AUTHOR(s):
%Luigi De Maria, Matteo D'Addazio, 2022
%

%% Main Code

epst = (x * exp((w-y)/z)) /...
       (1 + exp((w-y)/z))^2;

end